1.
Update your antivirus software and run a full scan. Antivirus software helps protect you from viruses and other suspect files that can spread by simply opening email. It performs regular system checks and detects when foreign files are trying to infiltrate your computer.
Norton and Kaspersky are the two standard-bearers in the industry, and can often be purchased for about $20.
Do not use Registry Optimizers or so called Clean Reg Tools. Most of these cause more harm than good in the future with your PC.
2.
Clear up some space on the hard drive. Deleting unnecessary files that your computer stores is an easy and simple way to free up memory and improve overall performance. To do this, access Disk Cleanup in Windows and delete Temporary Files on a Mac.

For Windows XP, Windows 7, and Vista: Click Start, click All Programs, click Accessories, click System Tools, and then click Disk Cleanup. If several drives are available, you might be prompted to specify which drive you want to clean.
For Mac: Go to Applications, click Utilities, and click Command Prompt. In the Command Prompt, type in "sudo rm -fr /tmp/*" and hit Enter. The temporary files that have been left behind on your computer will now be cleared.

3.
Uninstall useless programs or programs you rarely use. These can include games or media files that you or your children never use, as well as programs that have become redundant or obsolete.
For Windows: Follow the Start Menu to Settings and then to Control Panel. From this screen click on Add/remove programs or Uninstall a program and uninstall programs that are no longer needed.
For Mac: Kill unwanted or redundant programs by dropping the program files into the Recycle Bin.
Remember to empty your Recycle Bin after deleting these files because they are still on the hard drive and taking up space; just right click the icon and navigate to “Empty Bin.”
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4.
Download and install a spyware removing program. Spyware is a kind of malicious software installed on computers that tracks certain information without the user's knowledge. For both privacy implications and the health of your computer, it's best to delete spyware and other "malware."
You should be able to download anti-spyware software on your computer for free. Once downloaded, install the program and run it; if you are not an advanced computer user, read the directions.
5.
Defragment your hard drive. Fragmentation basically is when your computer's files get disorganized, hurting your computer's ability to lay data out sequentially. Defragmentation is the correction to this process.
For Windows: click on My Computer, then click Properties, then click the Tools tab. From this tab just click on the Defragment button and then run the program. You can set it up so that your computer automatically defragments at set intervals.
For Mac: Mac operating systems rarely, if ever, need to have their disk space defragmented.
6.
Check your RAM. RAM stands for Random Access Memory. These are the computer's memory chips, or how it stores information. If there are not enough of them the computer will use your hard disk to store intermediate results, but this is a much slower process. If this happens, the computer appears busy and is slow to write and read things from your hard disk. You can hear it, and the LED on the computer itself will light up.

Right click on the My Computer icon, then select properties, then read what is said on the 'General' tab. For Windows XP it should be 256 MB or more. If it is less then 1024 MB then it might be the problem.
If RAM is the real problem, your computer will be slow whenever you start a new application. If your computer is only slow when you turn on your computer or reboot, it is more likely that it is some other problem.
Before you rush out to get some additional RAM in a computer shop, make sure you know exactly what type fits in with your computer, and convince yourself that there are still empty slots (inside your computer) that can be used to put the RAM in.
For most people it is best to make a computer repair shop responsible for all this. Bring in your computer.